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Star Coin 1

About a third of the way through the stage, you'll see the first Star Coin just floating in the water near the top of the screen. Grab it!

Star Coin 2

Just after nabbing the first Star Coin, look for a pipe that's poking down from the top of the screen. Enter the pipe and you'll warp to a surface area with the second Star Coin.

Star Coin 3

Near the very end of the stage is the final Star Coin, nestled in a hard-to-navigate rock formation. It's easiest to grab the Star Coin as normal Mario or Mini Mario. The smaller versions of Mario can fit through the small gap and easily nab the coin. However, it is possible to grab the coin as a larger form of Mario. You'll need to be at the far right side of the screen just as the opening to the coin becomes available. Quickly swim left to grab the coin, then get ready to dodge the sharks that appear.

Red Coins

Just past the halfway checkpoint is the stage's Red Ring. Try to avoid triggering it until after it's about halfway through the screen. The time you have to collect the coins is limited, and if you hit the Red Ring early the coins won't appear on screen until late.